1. Technical Field
The present disclosure relates to semiconductor memory devices, and more particularly, to non-volatile SONOS memory devices and a method for manufacturing the same.
2. Discussion of Related Art
A typical semiconductor memory device operable with a microprocessor is a volatile type memory device. A volatile memory device loses stored data when power to the device is cut off. A semiconductor memory device may also be a non-volatile type. A non-volatile memory device retains stored information even when power is cut off.
A conventional non-volatile memory device is a charge-trapping device. An example of such a non-volatile memory device is a floating gate memory device. The floating gate memory device is a kind of a field effect device in which electric charges are stored in an isolated conductive material referred to as a floating gate.
The floating gate is made of a conductive material, and is formed between a semiconductor substrate and a gate electrode. The floating gate holds charges by programming. Since the floating gate is conductive, if a portion of a tunneling oxide layer isolating the floating gate and the semiconductor substrate suffers from defects, all charges stored within the floating-gate may be lost. Therefore, the floating gate memory device needs to have thick tunnel oxide layer as compared with a floating trap memory device (as described later) to maintain reliability. In this case, a complex peripheral circuit is required due to the high operating voltage necessitated by the increased thickness of the tunneling oxide layer. Consequently, disadvantages of the floating gate memory device include limitation of high-integration and high-power dissipation.
Another example of the charge-trapping devices is a floating trap memory device that stores charges in an insulation bulk trap of the field effect device. The floating trap memory device performs programming by storing charges in a trap formed within an insulation charge storage layer formed between the gate electrode and the semiconductor substrate. Examples of floating trap memory devices are metal-nitride-oxide-semiconductor (MNOS); metal-alumina-oxide-semiconductor (MAOS); metal-alumina-semiconductor (MAS); silicon-oxide-nitride-oxide-semiconductor (SONOS) memory cell, etc.
SONOS memory devices, an example of which is disclosed in U.S. Pat. No. 5,387,534 entitled “METHOD OF FORMING AN ARRAY OF NON-VOLATILE SONOS MEMORY CELLS AND ARRAY OF NON-VOLATILE SONOS MEMORY CELLS”, have been developed since the late 1960's.
Referring to FIG. 1, the non-volatile memory device according to the conventional art comprises source/drain regions 15 and 16, a gate insulation layer 17 including oxide-nitride-oxide layers (ONO) which are stacked sequentially on the semiconductor substrate 14, device isolation layers 19 and 20 and a gate electrode 18.
The non-volatile memory device operates by trapping electrons and holes in a nitride layer of the gate insulation layer 17. If a positive voltage is applied to the gate electrode 18, electrons tunnel from the semiconductor substrate to be captured in the trap within the nitride layer. As electrons are stored within the nitride layer of the gate insulation layer 17, a threshold voltage of the device rises to a programmed state.
To the contrary, if a negative voltage is applied to the gate electrode 18, electrons captured at the trap within the nitride layer of the gate insulation layer 22 tunnel to flow into the semiconductor substrate 14. At this time, holes of the semiconductor substrate 14 tunnel to be captured in the trap within the nitride layer. Accordingly, the threshold voltage of the device decreases to an erase state.
The non-volatile SONOS memory device can use a thinner gate insulation as compared to that used with the floating gate memory device because charges are stored in a deep level trap. In addition, the non-volatile SONOS memory device has a simple structure as compared with the floating gate memory device, so that the fabrication process is simple and high-integration is easily obtained.
In the conventional art, a high electrical field is necessary to generate hot carrier and to inject hot carrier into a nitride layer within the gate insulation layer when programming. For this reason, programming is achieved when the source/drain voltages are established over 7 voltages. Under this condition, current consumption is several hundreds μA. Accordingly, disadvantages of the conventional SONOS memory device include low pumping efficiency and excessive dimensions of the pumping circuit due to high voltage and excessive electric current dissipation during programming.